Hi @probinson1,

The message is accurate in that once you have graded a student submission, you cannot unpublish the course any longer.  Your school/institution may have settings in place to prevent students form accessing the course after the end date (or explicitly allowing them to have access, perhaps in a read only mode).  I would suggest that you reach out to the Canvas team at your school/institution (usually in the eLearning / IT / Digital Education areas) to see what options they do or do not make available to you and the students.

I hope this info helps!

-Chris